Circuits for increasing, decreasing and inverting a direct-current input voltage value to a different direct-current output voltage value include DC/DC converters. In view of the conversion efficiency and the amount of heat generated, a DC/DC converter has a better conversion efficiency and generates a smaller amount of heat than those of a regulator. They have a smaller device volume in comparison with a transformer. For these characteristics, they are used in work stations and personal computers, which are demanding in terms of the conversion efficiency, amount of heat generated, and device volume.
FIGS. 51A and 51B illustrate a structure of a conventional DC/DC converter 61. FIG. 51A illustrates a section for decreasing and outputting an input voltage. A power supply voltage is provided to a voltage input terminal. According to a pulse signal provided to a signal input terminal A and a signal input terminal B, NMOS transistors 50 and 51 are changed between an open state and a closed state. When the NMOS transistor 50 is closed while the NMOS transistor 51 is opened, a current is supplied to an LC section. The change in the supplied current over time is converted by an inductance L into a voltage, and a voltage increases faster at the terminal A than at an output terminal. Next, when the NMOS transistor 50 is opened while the NMOS transistor 51 is closed, a current is discharged from the LC section. The ratio between the supplied current and the discharged current determines the output voltage. When a period of time for which the NMOS transistor 50 is closed is longer than a period of time for which the NMOS transistor 51 is closed, the output voltage increases. When the period of time for which the NMOS transistor 50 is closed is shorter than the period of time for which the NMOS transistor 51 is closed, the output voltage decreases. For example, assume that the output voltage is 1.5 V when the period of time for which the NMOS transistor 50 is closed is equal to the period of time for which the NMOS transistor 51 is closed. Then, when the period of time for which the NMOS transistor 50 is closed is longer than the period of time for which the NMOS transistor 51 is closed, the output voltage is a voltage higher than 1.5 V. When the period of time for which the NMOS transistor 50 is closed is shorter than the period of time for which the NMOS transistor 51 is closed, the output voltage is a voltage lower than 1.5 V.
As illustrated in FIG. 51B, signals instructing the open/closed states of the NMOS transistors 50 and 51 are input through the signal input terminals A and B, and pulse signals input to the signal input terminals A and B are generated by a pulse generation section 55. The cycle and pulse width of an output pulse of the pulse generation section 55 are controlled by a control section 57. The control section 57 compares the voltage output from a reference voltage generation section 56 and the voltage at a monitor terminal of a voltage conversion section 54 with each other, and controls the cycle and pulse width of the pulse signal output from the pulse generation section so that the voltage at the monitor terminal is at a target voltage.
Recently, it has also been proposed to use a DC/DC converter in portable apparatuses such as a portable telephone or a PHS in order to prolong the life of a lithium ion battery. This is because there is a possibility that, by decreasing the output voltage of a lithium ion battery, which has a 3 V output voltage, to a voltage near 1 V by means of a DC/DC converter, and by operating an LSI used in a portable telephone at the voltage near 1 V, the power consumption of the LSI may be reduced.
However, in order to realize such a prolongation of battery life, it is necessary for the DC/DC converter to simultaneously solve the following problems (1) and (2).
(1) The conversion efficiency should not decrease even when converting the battery voltage to a voltage which is half or less than half of the battery voltage.
The output voltage of a lithium ion battery used in a portable telephone is 3 V. In order to reduce the power consumption of the LSI, it is necessary to efficiently decrease the output voltage of the lithium ion battery to 1 V. However, when it is attempted to effect such a voltage decrease using the conventional DC/DC converter 61 (FIG. 51B), the conversion efficiency decreases. This is because the power consumption of a control system circuit 58 in the conventional DC/DC converter 61 is large. For example, when the power supply voltage is 1 V, the power consumption of the LSI is about 10 mW, whereas the power consumption of the control system circuit 58, which includes the pulse generation section 55, the control section 57 and the reference voltage generation section 56, is about 100 mW. Thus, a cause of the decrease in conversion efficiency is that when the power supply voltage is low, the power consumption of the control system circuit 58 of the DC/DC converter 61 is larger than the power consumption of the LSI.
(2) The voltage conversion section should have an efficiency of 90% or more.
In the conventional DC/DC converter 61, the decrease in efficiency of the voltage conversion section 54 occurs due to currents flowing through the NMOS transistors 50 and 51. In the voltage conversion section 54, a doubled decrease in efficiency results for one cycle. This is because the NMOS transistors 50 and 51 are opened in a single cycle in the voltage conversion section 54.
Moreover, problems associated with the on-chip technique include (3) below.
(3) On-chip implementation should be easy.
In the conventional DC/DC converter 61, the value of the inductor 52 is about 100 .mu.H. However, it is difficult to form an inductor having such a large value on a silicon substrate. This is because it is only possible to form an inductor of about 200 nH, at best, on a silicon substrate. When an inductor of about 100 .mu.H is used, there is a possibility that radiating electromagnetic noise may cause malfunctioning of other LSIs.
Moreover, in order to realize a conversion efficiency of 80% or more in the conventional DC/DC converter 61, a resistance while the NMOS transistors 50 and 51 are closed (ON resistance) needs to be about 0.1 m.OMEGA.. However, it is difficult to form a switch having such a small ON resistance on a silicon substrate. This is because it is only possible to form a switch whose ON resistance is about 500 m.OMEGA., at best, on a silicon substrate. When a switch having an ON resistance of about 500 m.OMEGA. is used, the conversion efficiency decreases to 60% or less.
Thus, any of the problems (1)-(3) cannot be solved with the conventional DC/DC converter 61.
An object of the present invention is to simultaneously solve the above-described problems (1)-(3) while providing a voltage converter which realizes a high-efficiency voltage conversion even when a small output current is output.
Moreover, the present invention is a basic invention of a power supply apparatus suitable for a low power LSI. The present invention has an objective to provide a power supply apparatus which has the following characteristics: (1) having substantially no energy loss; (2) being capable of generating various types of voltage waveforms; and (3) being suitable as a power supply for an LSI.
Furthermore, the present invention has an objective to provide a semiconductor integrated circuit comprising a power supply apparatus, including an LC resonance circuit, and at least one circuit block to which a power supply voltage is supplied from the power supply apparatus, wherein it is possible to reduce noise generated by the operation of the LC resonance circuit.